Method for separating air by cryogenic distillation and installation therefor

ABSTRACT

The invention concerns a method for separating air by cryogenic distillation using an apparatus comprising a medium pressure column ( 9 ) and a low pressure column ( 11 ) thermally communicating, which consists in cooling an amount of compressed and purified air V in an exchange line ( 10 ) to a cryogenic temperature and conveying at least part of it to the medium pressure column, conveying oxygen- and nitrogen-enriched flows (LR, LP) from the medium pressure column to the low pressure column and drawing nitrogen- and oxygen-enriched flows ( 35, 23 ) from the low pressure column. The invention is characterized in that the medium pressure column operates between 6 and 9 bar abs and the ratio between the total amount of air V entering the exchange line and the total volume of the exchange line ranges between 3000 and 6000 Nm 3 /h/m 3 .

[0001] The present invention relates to a process for separating air bycryogenic distillation and to an installation for implementing thisprocess.

[0002] In general, the objective of an engineer creating a process forseparating air is to minimize the expenditure of energy.

[0003] It is well known to use, for producing oxygen with low energy, adouble air separation column which is applied, in particular, on the onehand, so as to minimize the delivery pressure of the air compressor, byreducing the head losses in the exchange line and reducing thetemperature difference at the main vaporizer, and, on the other hand, tomaximize the oxygen extraction efficiency, by reducing the temperaturedifference in the exchange line, by choosing a high number oftheoretical distillation trays and by installing a sufficient number ofsections of structured packings or trays.

[0004] Thus, low-pressure columns have four sections of structuredpackings or trays, including two sections between the bottom of thelow-pressure column and an intake for rich liquid, this being anoxygen-enriched liquid taken from the bottom of the medium-pressurecolumn. These two sections are necessary for providing high-performancedistillation in the bottom of the low-pressure column. Thus, themedium-pressure columns have four sections of structured packings ortrays, including two sections between the liquid air intake and thepoint of withdrawal of lean liquid.

[0005] The purified and compressed air sent to the columns cools in anexchange line comprising which would normally have a volume of more than200 m³, and therefore with a ratio of the total air volume sent to theexchange line to the volume of the exchange line that would beapproximately 2000 Sm³/h/m³ in the case of the example described below.

[0006] The refrigeration required for the distillation is frequentlyprovided by an air stream sent to a blowing turbine that feeds thelow-pressure column and/or an air stream sent to a Claude turbine. Theratio of the quantity of air sent to the exchange line to the volumesent to the blowing turbine would normally be between 5/1 and 15/1 inthe case of the example described below.

[0007] In certain cases when energy is not expensive, or even free, itis profitable to reduce expenditure on equipment, while increasingenergy requirements.

[0008] It is an object of the present invention to reduce the investmentcost of an air separation installation and to increase its energy byreducing the size of the exchangers (and therefore increasing the headlosses and the temperature differences in the exchange line, andincreasing the temperature difference at the main vaporizer), byreducing the size of the distillation columns (by minimizing the numberof theoretical trays and the number of sections of packings or trays)and by reducing the size of the refrigerating turbine (by increasing itsintake temperature in order to reduce its output).

[0009] The quantity of air V sent to the exchange line comprises all theair sent to the distillation unit and the possible streams of air thatare expanded and then vented to atmosphere.

[0010] A section of structured packings is a section of structuredpackings between a fluid inlet or outlet.

[0011] The structured packings are typically of the cross-corrugatedtype, but they may have other geometries.

[0012] The subject of the present invention is a process for separatingair by cryogenic distillation using an apparatus comprising amedium-pressure column and a low-pressure column that are thermallycoupled, in which a quantity of compressed and purified air V is cooledin an exchange line down to a cryogenic temperature and is sent at leastpartly to the medium-pressure column, oxygen-enriched andnitrogen-enriched streams are sent from the medium-pressure column tothe low-pressure column and nitrogen-enriched and oxygen-enrichedstreams are withdrawn from the low-pressure column, characterized inthat the medium-pressure column operates between 6 and 9 bar absoluteand the ratio of the total quantity of air V entering the exchange lineto the total volume of the exchange line is between 3000 and 6000Sm³/h/m³.

[0013] According to other optional aspects:

[0014] the maximum temperature difference at the cold end of theexchange line is 10° C.;

[0015] the maximum temperature difference at the warm end of theexchange line is 3° C.;

[0016] the maximum temperature difference at the start of liquid oxygenvaporization in the exchange line is 3° C.;

[0017] the maximum temperature difference at the end of liquid oxygenvaporization in the exchange line is 10° C.;

[0018] an oxygen-enriched liquid is sent from the low-pressure column toa sump reboiler where it partially vaporizes by heat exchange with anitrogen-enriched gas coming from the medium-pressure column, thereboiler having a ΔT of at least 2.5 K;

[0019] a portion of the compressed and purified air is sent into ablowing turbine, having an inlet temperature of between −50 and −90° C.;

[0020] the ratio of the quantity of air V to the volume of air sent tothe blowing turbine is between 20 and 40;

[0021] the medium-pressure column contains two or three sections ofstructured packings and/or the low-pressure column contains threesections of structured packings;

[0022] at least one liquid stream is withdrawn from a column, optionallypressurized and vaporized in the exchange line;

[0023] the medium-pressure column operates at between 6.5 and 8.5 barabsolute;

[0024] the head losses in the exchange line are greater than 200 mbarfor a waste nitrogen stream coming from the low-pressure column;

[0025] the head losses in the exchange line are greater than 250 mbarfor the lower-pressure air stream;

[0026] the ratio of the quantity of air V to the volume of air D isbetween 20/1 and 40/1;

[0027] i) a liquid-air expansion turbine is fed by all or part of astream of liquid air output by the exchange line; and/or

[0028] ii) a refrigeration set or chilled water produced by arefrigeration set (which may be the same water circuit as that used forcooling the air at the inlet of the purification unit) cools the airoutput by an air supercharger and/or the air at the lowest pressure;and/or

[0029] iii) an increased ratio of air is sent to the blowing turbine insuch a way that the ratio of the quantity of air V sent to the exchangeline to the volume of air D sent to the blowing turbine is less than20/1;

[0030] the purity of the oxygen is between 85 and 100%, preferablybetween 95 and 100%.

[0031] the oxygen extraction efficiency is between 85 and 100%

[0032] The subject of the invention is also an air separationinstallation for producing air gases using a process described above,comprising the medium-pressure column containing two or three sectionsof structured packings and/or the low-pressure column containing threesections of structured packings.

[0033] Optionally, the installation may include an argon column fed fromthe low-pressure column.

[0034] A blowing turbine expands air and sends at least one portionthereof to the low-pressure column of a double column.

[0035] The invention will now be described with reference to the figure,which is a diagram of an installation for implementing the processaccording to the invention.

[0036] A 475000 Sm³/h stream 1 at 7 bar absolute, coming from apurification unit (not illustrated), is divided into three. A firststream 3 is supercharged in the supercharger 5 up to the pressurerequired to vaporize the liquid oxygen for example. The high-pressureair HP AIR 7 is sent to the exchange line 10 but does not reach the coldend, being cooled down to −160° C., expanded, liquefied and sent to thetwo columns 9 and 11, namely the medium-pressure column and thelow-pressure column, respectively, of an air separation double column.

[0037] A second, non-supercharged, stream MP AIR 13 is also sent to theexchange line 10, through which it partly flows until reaching −140° C.before being sent to the bottom of the medium-pressure column 9.

[0038] A 20000 Sm³/h third stream 15 is sent to a supercharger 17,partly cooled in the exchange line, and is expanded in a blowing turbine19, with an inlet temperature of −80° C., before being sent to thelow-pressure column 11. The ratio of the volume of air sent through theblowing turbine 19 to the quantity of air sent to the exchange line is24/1.

[0039] The head losses in the exchange line 10 are about 300 mbar in thecase of the air stream 13 at the lowest pressure and about 250 mbar inthe case of the waste nitrogen 35.

[0040] The exchange line 10 has a volume of 125 m³, thus the ratio ofthe quantity of air sent to the exchange line 10 (stream 1 or volume V)to the volume of this exchange line 10 (=number of bodies×totalwidth×total stack×total length) is 3800 Sm³/h/m³.

[0041] The double column is a conventional apparatus except as regardsits dimensions and the number of theoretical trays of the columns, sincethe medium-pressure column contains 40 theoretical trays and thelow-pressure column 45 of them, and as regards the temperaturedifference in the case of the reboiler 21, which is greater than 2.5° C.

[0042] Conventionally, oxygen-enriched liquids (rich liquid RL) andnitrogen-enriched liquid (lean liquid LL) are sent from themedium-pressure column to the low-pressure column after subcooling inthe exchanger SC and expansion in a valve.

[0043] The low-pressure column 11 contains three sections of structuredpackings, comprising a sump section I between the bottom of the columnand the rich liquid intake (which is conjoint with the blown airintake), a section II between the rich liquid intake and the liquid airintake and a section III between the liquid air intake and the leanliquid intake.

[0044] The medium-pressure column 9 contains three structured packings,comprising a sump section I between the bottom of the column and theliquid air intake, a section II between the liquid air intake and thelean liquid outlet LL and a section III between the lean liquid outletLL and the medium-pressure nitrogen outlet 31. Of course, if there is nowithdrawal of liquid nitrogen or gaseous nitrogen, the medium-pressurecolumn contains only two sections, section III being omitted.

[0045] The sump reboiler 21 of the low-pressure column 11 is in factincorporated with the medium-pressure column 9 and is warmed by a streamof medium-pressure nitrogen of this column 9. A stream of liquid oxygen23 coming from the bottom of the low-pressure column 11 is pumped inorder to overcome the hydrostatic head and arrives in the reboiler 21where it partially vaporizes, a gas stream 25 being sent back to thelow-pressure column below the exchange means I and a liquid stream 27being sent to the pump 29, where it is pressurized up to its usepressure.

[0046] The pumped stream 27 vaporizes in the exchange line 10.

[0047] A stream of liquid nitrogen 31 is withdrawn as top product fromthe medium-pressure column 9 above section III, pumped and alsovaporizes in the exchange line 10.

[0048] The pressure of the liquid nitrogen and the pressure of theliquid oxygen may take any value, provided that the exchange line 10 isdesigned according to the maximum pressure of the air required forvaporization.

[0049] It will be understood that the invention also applies to the casein which a single stream of liquid vaporizes in the exchange line 10, orno liquid withdrawn from a column vaporizes in the installation.

[0050] Instead of vaporizing against air, the stream or streams ofliquid may vaporize against a stream of cycle nitrogen.

[0051] Alternatively, the liquid stream or streams may vaporize in adedicated exchanger serving only to vaporize the liquid stream orstreams against a stream of air or a stream of cycle nitrogen.

[0052] The process may also produce liquid oxygen and/or liquid nitrogenand/or liquid argon as final product(s).

[0053] Gaseous nitrogen 33, 35 may be withdrawn from the medium-pressurecolumn 9 and/or from the low-pressure column 11.

[0054] The gaseous nitrogen 35 warms in the subcooler SC.

[0055] Alternatively or in addition, a stream of gaseous oxygen (notillustrated) may be withdrawn as final product from the low-pressurecolumn 11. Optionally, this stream may be pressurized in a compressor.

[0056] A stream of medium-pressure gaseous nitrogen MP NG 33 and astream of low-pressure waste nitrogen 35 are warmed in the exchange line10. The stream WN may serve to regenerate the air purification system ina known manner and/or may be sent to a gas turbine.

[0057] A process as described is used to produce 99.5% pure oxygen HP OGwith a yield of more than 97%. This oxygen serves typically in agasifier supplied with a fuel such as natural gas.

[0058] In the installation, the low-pressure column 11 may be alongsidethe medium-pressure column 9, as in the example, or else above thelatter.

[0059] To produce a stream of liquid oxygen and/or liquid nitrogenand/or liquid argon and/or to reduce the pressure levels, especially thepressure of the HP AIR 7, the refrigeration required may be provided byusing:

[0060] i) a liquid-air expansion turbine fed completely or partly withthe liquid air stream HP 7 output by the exchanger (10); and/or

[0061] ii) a refrigeration set or chilled water produced by arefrigeration set (which may be the same water circuit as that used forcooling the air at the inlet of the purification unit) in order to coolair output by the air supercharger 5 and/or the air output by thesupercharger 17 and/or the MP 13; and/or

[0062] iii) by sending an increased ratio of air to the blowing turbine19 in such a way that the ratio of the quantity of air V sent to theexchange line to the volume of air D sent to the blowing turbine is lessthan 20/1.

[0063] These means for generating refrigeration may also be employed inthe case in which no liquid is produced.

[0064] The superchargers 5, 17 and/or the main compressor (notillustrated) may be driven by electricity, by a steam turbine and/or bya gas turbine.

[0065] The turbine 19 may have a dedicated supercharger or a generator.

[0066] The installation may also include conventional components, suchas a Claude turbine, a hydraulic turbine, a medium-pressure orlow-pressure nitrogen turbine, one or more argon production columns, amixing column fed with air and oxygen from the low-pressure column, acolumn operating at an intermediate pressure, for example one fed withthe rich liquid and/or with air, a double-reboiler or triple-reboilerlow-pressure column, etc.

1-15 (cancelled).
 16. A process for separating air by cryogenic distillation comprising: a) coupling a medium-pressure column and a low-pressure column, wherein said medium-pressure column operates between 6 and 9 bar absolute; b) cooling a quantity of compressed and purified air in an exchange line down to a cryogenic temperature, wherein the ratio of the total quantity of air entering the exchange line to the total volume of the exchange line is between 3000 and 6000 Sm³/h/m³; c) sending at least part of the cooled air to the medium-pressure column; d) sending oxygen-enriched and nitrogen-enriched streams from the medium-pressure column to the low-pressure column; and e) withdrawing nitrogen-enriched and oxygen-enriched streams from the low-pressure column.
 17. The process as claimed in claim 16, wherein an oxygen-enriched liquid is sent from the low-pressure column to a sump reboiler where it partially vaporizes by heat exchange with a nitrogen-enriched gas coming from the medium-pressure column, the reboiler having a temperature differential of at least 2.5° C.
 18. The process as claimed in claim 16, wherein a portion of the compressed and purified air is sent into a blowing turbine, having an inlet temperature of between −50 and −90° C.
 19. The process as claimed in claim 18, wherein the ratio of the quantity of air to the volume of air sent to the blowing turbine is between 20 and
 40. 20. The process as claimed in claim 16, wherein the medium-pressure column contains two sections of structured packings.
 21. The process as claimed in claim 16, wherein the medium-pressure column contains three sections of structured packings.
 22. The process as claimed in claim 20, wherein the low-pressure column contains three sections of structured packings.
 23. The process as claimed in claim 16, wherein at least one liquid stream is withdrawn from at least one of the medium-pressure column and the low-pressure column and vaporized.
 24. The process as claimed in claim 16, wherein the medium-pressure column operates between 6.5 and 8.5 bar absolute.
 25. The process as claimed in claim 16, wherein the head losses in the exchange line are greater than 200 mbar for a waste nitrogen stream coming from the low-pressure column.
 26. The process as claimed in claim 16, wherein the head losses in the exchange line are greater than 250 mbar for the lower-pressure air stream.
 27. The process as claimed in claim 16, wherein the ratio of the quantity of air to the volume of air is between 20:1 and 40:1.
 28. The process as claimed in claim 16, further comprising: f) feeding a liquid-air expansion turbine by all or part of a stream of liquid air output by the exchange line; g) cooling the air output by an air supercharger and the air at the lowest pressure with a refrigeration set or chilled water produced by a refrigeration; and h) increasing the ratio of air sent to the blowing turbine in such a way that the ratio of the quantity of air sent to the exchange line to the volume of air sent to the blowing turbine is less than 20:1.
 29. The process as claimed in claim 16, wherein the purity of the oxygen is between 85 and 100%.
 30. The process as claimed in claim 29, wherein the purity of the oxygen is between 95 and 100%.
 31. The process as claimed in claim 16, wherein the oxygen extraction efficiency is between 85 and 100%.
 32. An air separation installation apparatus for producing air gases using a process as claimed in claim 16, comprising the medium-pressure column containing two or three sections of structured packings and the low-pressure column containing three sections of structured packings.
 33. The installation as claimed in claim 32, further comprising an argon column fed from the low-pressure column. 